Among the most insidious features of drug addiction is the propensity to relapse after periods of prolonged abstinence. The enduring vulnerability to relapse reflects long-term cellular changes in brain circuitry. Experiments in animal models of drug-seeking reveal adaptations in the series circuit consisting of the glutamatergic projection from the prefrontal cortex (RFC) to the nucleus accumbens and the GABA/enkephalin projection from the accumbens core (NAcore) to the ventral pallidum (VP). Identifying these projections as potentially critical in animal models of addiction has led to a search for enduring cellular changes in the PFC, NAcore and VP that underlie drug-seeking. This proposal will use a reinstatement model in rats trained to self-administer cocaine to characterize adaptations in this circuit that underlie drug-seeking. It is hypothesized that the PFC - NAcore - VP projection is a series circuit that undergoes pathological changes in protein expression and function as a result of repeated drug exposure and withdrawal. To test the veracity of this hypothesis candidate proteins will be manipulated in order to reduce cocaine- or stress-induced reinstatement of drug-seeking. In aim #1 the role of dopamine receptor subtypes in the PFC in regulating reinstatement will be examined by a combination of drug microinjection and microdialysis. In aims #2 and #3 the role of pre- and postsynaptic proteins that regulate glutamate transmission in the NAcore will be examined, including the cystine-glutamate exchanger, mGluR2/3, Homer and actin. A combination of microdialysis and injection of reagents to manipulate protein levels such as virus and Tat fusion proteins will be used. Finally, in the VP the role of mu opioid regulation of GABA release in drug-seeking will be characterized. The overarching goal of the proposal is to identify novel molecular targets for pharmacotherapeutic intervention in cocaine craving and relapse.